General-purpose gasoline engines are reciprocating piston gasoline engines with broad applications, except applications in vehicle and aerocraft; generally, those with power within 20 kW are called as small general-purpose gasoline engines; and they have high generality and advantages of small volume, small mass, simple and convenient use and operation, and low price. As one of power sources, the general-purpose gasoline engine is used for driving general-purpose machinery, including agriculture and forestry plant protection machinery, garden machinery, power generation sets and construction machinery and the like. As a kind of power machinery using gasoline as fuel, the general-purpose gasoline engine consists of a cylinder, a crank and connecting rod mechanism, a valve system, an fuel supply system, a lubrication system and an ignition system and the like; cylinder head is provided with an intake passage and an exhaust passage and forms a combustion chamber surface together with the cylinder; therefore it is a very important component of the gasoline engine. The mixture of gasoline served as fuel and air combusts in the combustion chamber, a lot of heat is generated and used as driving energy. Parameters of the combustion chamber are essential conditions for guaranteeing the high-efficiency operation of the gasoline engine; simultaneously, the continuous operation of the combustion chamber needs the coordination of the intake system and the exhaust system, if the resistance to intake and exhaust is relatively large, or closure is not tight or accurate, or the action is not smooth, the normal operation of the gasoline engine may be influenced.
Heat generated in the combustion chamber influences the mechanical property of the cylinder head inevitably, particularly for local areas around the intake valve and the exhaust valve, if they cannot be cooled in time, thermal deformation of these areas may be caused to influence the tightness of the intake valve and the exhaust valve, so that normal intake and exhaust are influenced, the working status of the gasoline engine becomes poor, the power is decreased, and the emission is influenced. In the prior art, in order to solve the cooling problem, as a cooling measure for the cylinder head, arrangement of cooling fins on the outer surface is generally adopted, the heat inside the combustion chamber is conducted to the cooling fins through the cylinder head, and taken away through air convention to achieve the purpose of cooling. As the nose bridge region formed on the combustion chamber surface between the intake valve and the exhaust valve is far away from the surface of the cylinder head, most of head cannot be taken away by the cooling fins in time, after the gasoline engine operates for a long time, the foregoing thermal deformation may be caused, so that the power and the emission of the gasoline engine are influenced.
The valve system of the gasoline engine is an essential component to guarantee normal operation of the gasoline engine, the opening and closing of the intake valve and the exhaust valve are respectively controlled by driving elements such as rocker arm assembly; the intake valve and the exhaust valve are respectively provided with rocker arm assemblies used for driving the opening and closing of the valves; the opening and closing of the intake valve and the exhaust valve are matched with suction, combustion and exhaust inside the combustion chamber to maintain normal operation of the gasoline engine. In the prior art, rocker arms and valves are arrayed in rectangular, that is, rocker arms are paratactic and approximately parallel, the intake and exhaust passages are matched with the valves, and a certain space has to be reserved to avoid influencing the quality of the intake and exhaust. It can be seen that the existing arrangement of valves not only occupies a large space, but also mismatches with the intake and exhaust direction of the intake passage and the exhaust passage, so that the power of the gasoline engine is decreased, and the emission is influenced; particularly for a structure with obliquely arranged valves, the occupation space may be further increased, and the arrangement of other components (such as carburetor/air filter and other parts) may be influenced.
Therefore, it is necessary to improve the existing general-purpose gasoline engine to make it have a combustion chamber structure with smaller surface to volume ratio, so that the cooling effect is good, the intake and exhaust resistance is reduced, the intake and exhaust valves have excellent tightness effect, the arrangement of other components is benefited, the operation efficiency of the gasoline engine is improved, the fuel is saved, and the emission is reduced.